1. Field of the Invention
This invention relates to an apparatus for controlling a hydraulic turbine. More particularly, this invention relates to an apparatus for controlling the position of adjustable wicket gates and adjustable runner blades of a hydraulic turbine
2. Description of the Prior Art
In the prior art, hydraulic turbines are well known having adjustable runner blades and adjustable wicket gates. Historically, apparatus for controlling the position of the wicket gates and runner blades has been predominantly mechanically actuated Examples of such may be found in U.S. Pat. No. 2,331,507 to Ring dated Oct. 12, 1943 and U.S. Pat. No. 2,132,504. Electro-hydraulic apparatus have been developed to perform the wicket gate and turbine blade adjustments previously performed by mechanical systems. An example may be found in U.S. Pat. No. 4,137,004. Such hydraulic systems use electrical means for determining the actual position of hydraulic turbine wicket gates or runner blades. The actual position of the blades is compared with a desired position and an electrical signal is sent to a hydraulic circuit for operating a hydraulic motor which affects adjustment of the blades or wicket gates. The hydraulic motor is commonly a cylinder having a reciprocating piston with the piston connected through linkage, well known in the art, for affecting adjustment of the blades and wicket gates.
In prior art positioning apparatus, the hydraulic circuits were controlled by means of a solenoid valve or a servovalve. A solenoid valve is a valve which, for example, may assume one of three states. That is, the valve has a forward state, a neutral state and a reverse state. When a proper electronic signal is received by the valve, the valve assumes either of the three states such that when in the forward state, a hydraulic circuit is completed tending to urge the blades or wicket gates toward an open position. When the valve assumes a neutral state, no hydraulic circuit is completed and the blades and wicket gates are not permitted to move. When in the reverse state, the valve completes a hydraulic circuit such that the blades or wicket gates are caused to move toward a closed position. Solenoid valves may conveniently be referred to as digitally responsive. That is, when the valve is in the forward or reverse state, the rate of flow permitted through the valve is constant. As a result, the valve is either fully on (in a reverse or forward state) or fully off. The use of such valves to control the position of hydraulic turbine runner blades presents several problems in the art. Namely, the circuit which provides hydraulic fluid to the hydraulic motor which positions the runner blades is susceptible to hydraulic leakage such that the blades tend to wander from their desired position. As a result, the solenoid valve must be constantly turned on and off between the forward and reverse states to constantly move the runner blades into their desired position to make up for the loss of hydraulic fluid. Such a method of operation is commonly referred to as a "bang-bang" approach where the hydraulic turbine runner blades are permitted to drift away from their desired position by a predetermined tolerance at which point the solenoid valve is turned on in either the reverse or forward state to urge the blades back to their desired position. When the blades are within the predetermined tolerance range, the solenoid valve is in its neutral state. Accordingly, a turbine positioner which operates on a single solenoid valve must permit some degree of inaccuracy in the position of the turbine blades.
Another method of completing a hydraulic circuit to affect positioning of turbine blades and wicket gates is the use of a servovalve which has three states (reverse, forward and neutral) such that the circuit operates in the same manner as the solenoid valve when the servovalve assumes one of the three states. The servovalve system differs from the solenoid system in that a servovalve is proportionally responsive. For example, the servovalve receives a signal of varying current. When the current is positive, the servovalve is urged to the forward state. When the current is negative, the servovalve is moved to the reverse state and when no current is admitted to the servovalve, the servovalve assumes the neutral state. Unlike a solenoid valve, a servovalve can regulate the amount of flow through the valve when in the reverse and forward state. That is, the servovalve is responsive to the absolute valve of the current signal such that the flow rate through the valve is adjustable from zero to some maximum flow rate when in either of the forward or reverse state. A problem which has arisen in operation of a hydraulic circuit using a servovalve for controlling hydraulic turbine blades and wicket gate is the size of the valve results in inaccuracies in adjustments of the blades. The maximum flow rate through a servovalve is selected so that a complete stroke of turbine runner blades may be achieved within a desired amount of time. However, in the case where a turbine blade wanders as a result of loss of hydraulic fluid in the circuit, only a small flow rate is needed to affect realignment of the blades. At low flow rates, such large capacity servovalves are inaccurate and unpredictable.
It is an object of the present invention to provide an apparatus for controlling the operation of a hydraulic turbine which employs a hydraulic circuit which is responsive to fluctuations of turbine blades as a result of loss of hydraulic fluid but which is of sufficient capacity to operate the turbine blades through the entire desired stroke. A further object of the present invention is to provide an apparatus for controlling a hydraulic turbine which comprises a solenoid valve and a servovalve arranged in parallel with the solenoid valve operating the control of the turbine blades to within a predetermined tolerance with the servovalve completing positioning of the turbine blades.